Full color imaging systems are well known in the art. In the past, they have been based extensively upon silver halides (see, for example, Krause, P., Imaging Processes and Materials, 8th ed.; Sturge, J.; Walworth, V.; Shepp, A., Eds.; Van Nostrand Reinhold; New York, 1989; pp. 110-34); dry silver (e.g., U.S. Pat. Nos. 2,772,971, 2,78,625, 2,623,823, and 2,594,917); and microencapsulated (e.g., U.S. Pat. No. 4,576,891) technologies for image reproduction. Because of their extreme sensitivity to light, silver halide systems have been the most widely used.
The use of barrier layers to separate the chemistry of imaging layers in full color silver halide systems has been employed in the past (see, for example, Krause, P., Imaging Processes and Materials, 8th ed., et al., p. 120). In the silver halide systems, the principal role of the barrier layer has been to keep the chemistries of the individual imaging layers separated and thus avoid the poor quality reproduced images which result from interference or "cross-talk" between the chemistries of the individual layers. Gelatin based barrier layers have typically been employed in silver based systems.
Although silver halide based systems have been satisfactory for their intended use, the photographic industry has searched for alternatives. Silver based systems can sometimes be inconvenient to use, particularly where wet processing is utilized, and furthermore, the cost of silver can be prohibitively expensive.
To that end, photothermographic systems have become known in the art. As the term implies, photothermographic systems rely upon heat to develop a radiation generated latent image. One type of construction for these photothermographic systems incorporates one or more nitrate salt/leuco dye based light sensitive imaging layers. As is known, the nitrate salt undergoes decomposition upon application of heat (e.g., 80.degree. to 90.degree. C.) to generate various intermediate vaporous products, one of which will oxidize the leuco dye so that the dye can then express its specific color in the reproduced image. U.S. Pat. Nos. 4,386,154, 4,460,677, 4,370,401, and 4,394,433 disclose photothermographic nitrate ion based imaging systems. Japanese Patent Nos. 77/025,330, 77/004,180, and 79/001,453 disclose nitrate ion oxidation mediate photothermographic materials and U.S. Pat. Nos. 4,336,323 and 4,373,020 disclose bleachable nitrate containing systems. However, none of the foregoing teach the use of barrier layers to separate imaging layers or their use in the construction of a full color imaging system. No problems which would require barrier layers in such photothermographic systems are known to have been reported to date in the trade or patent literature.
Because of the practical advantages of using photothermographic systems over silver based systems, there has been a demand in the photographic industry for improved full color photothermographic systems which utilize a plurality of light sensitive layers. The use of a plurality of light sensitive layers, each layer containing a different color system, is advantageous because a more saturated full color image can be reproduced. However, the industry has found itself lacking the availability of such multi-light sensitive layered photothermographic systems. There is also an inherent difficulty that exists in developing a suitable barrier layer which would be necessary to separate the individual light sensitive layers.
The difficulty in developing a suitable barrier layer for a non-silver based, imaging system resides in the fact that such a barrier layer must possess properties beyond those which are necessary in barrier layers of silver based imaging systems. In a photothermographic system containing a plurality of light sensitive layers, a barrier layer must have a unique and very careful balance of several properties.
Additional considerations are necessary when developing an effective photothermographic imaging system for at least a couple of important reasons, all of which are due in part to the complicated chemical nature of the light sensitive layers in photothermographic systems. To begin with, as explained earlier, the decomposition of the nitrate salt present in the light sensitive layer results in various intermediate vaporous products. The various vaporous intermediates which do not participate in the oxidation of the leuco dye and thus, are not absorbed by the dye, can accumulate to the point where pressure builds up in the individual light sensitive layers. If the vapors are not released, an undesirable "blistering" of the final multi-colored image will result. Thus, one is confronted with the situation that whereas a barrier coating must be somewhat impermeable to the oxidizing vaporous intermediates in order to prevent cross-talk from occurring between the individual light sensitive layers, the barrier coating must also have some degree of permeability so that vapors do not accumulate in the individual layers.
Additionally, the light sensitive layers and the barrier layers must be substantially insoluble in one another. That is, when the barrier layer is coated on top of a dried light-sensitive layer or vice versa, not more than 5 wt% of the critical imaging ingredients (e.g., dye, photosensitive agent, nitrate salt, etc.) of the bottom layer should leach, migrate, be extracted into, intermix with or otherwise be transferred to the top layer.
In view of the foregoing, it is clear that the development of a suitable barrier coating for use in a photothermographic system containing multiple light sensitive layers poses special considerations not previously required in the development of more traditional silver based imaging systems. Consequently, there has existed in the past a void in the photographic industry for this kind of product. It was against this background that a search was begun for a suitable photothermographic system which would overcome the above mentioned difficulties and fulfill the needs of the industry.